Microbial fuel cells (MFCs) are devices capable of directly transforming chemical energy to electrical energy via electrochemical reactions involving biochemical pathways. A typical microbial fuel cell consists of anode and cathode compartments separated by a cation specific membrane. In the anode compartment, fuel is oxidized by microorganisms or by purified isolated enzymes, generating electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, and the protons are transferred to the cathode compartment through the membrane.
Microbial based fuel cells exploit whole organism metabolism for catalyzing oxidation/reduction of metabolites (fuel) at the anode or cathode whereas enzyme based fuel cells utilize purified enzymes as catalysts.
Microbially catalyzed systems are generally robust, capable of completely oxidizing the fuel and have a long lifetime. However, these systems are limited by low current and power densities.
Enzymatically catalyzed systems are generally well defined systems showing higher current and power densities. However, enzyme based bio-fuel cells suffer from poor stability and short-term operational capacity, since enzyme stability and activity decline outside of the cellular milieu and since enzyme coupling with inorganic surfaces is poor. Further, enzymatic fuel cells still lack sufficient power density to render them applicable in “real world” applications, the main reason being their slow metabolic rates and lack of efficient means for transferring the electrons generated by substrate oxidation to the extracellular environment (namely, the anode).
Because living cells have metabolic pathways for oxidizing a wide variety of substrates, many biodegradable organic matter may be used in an MFC to generate power, including simple molecules such as carbohydrates and proteins and complex mixtures of organic matter including alcohols, fatty acids and carbohydrates present, for example, in waste waters. Thus, MFCs are ideal for renewable bioelectricity generation from biomass.
In the design of whole-cell-based microbial fuel cells, a significant step is the transfer of electrons back and forth between the microbe cell and the electrode. Optimized electron flow is currently hampered by the inefficient coupling between the electron acceptor (usually a gold electrode) and the electron donor (microorganism). Bare gold is a poor electron acceptor from electrogenic microorganisms. Also, the active site of most enzymes is embedded deeply within the protein matrix, thus negating the possibility of direct communication with electrodes. In most cases, in order to overcome these problems, an artificial mediator for electron transfer is used. These electron shuttles (such as thionine, methyl viologen, methyl blue, humic acid and neutral red) enable microorganisms to generate electrochemically active reduced products. Unfortunately, systems including mediators are not easily optimized and most of the available mediators are expensive and toxic.
Direct electron transfer was discovered in several microorganisms (termed exoelectrogens or electricigens). Thus it became possible to construct an MFC without requiring external mediators, by using exoelectrogens. However, the use of mediators is still appealing since it affords anodes with lower potentials, which increases the overall electromotive force (EMF) of a given MFC.
The limitations of the existing MFCs prevent this important technology from being used in “real world” applications.
There is a need for a self-sustaining and renewable system for efficient extraction of electrons from organic compounds.